Lignite-fired electric power facilities produce a byproduct of the combustion process commonly referred to as clinkers. A clinker is a conglomeration of molten ash which is produced by the burning lignite. The clinkers accumulate along the boiler hopper walls and continuously drop to the bottom into a cooling pool of water. The cooling pool serves to fragment the clinkers and to flush them out of the hopper.
As clinkers build up on the hopper walls, the efficiency of the furnace is reduced due to the poor heat transfer properties of the clinkers. In addition, the clinkers that fall to the bottom of the hopper can hinder the operation of the flushing apparatus or block it completely. In order to circumvent this problem, the hoppers incorporate a grinder in the flushing apparatus which pulverizes the clinkers that are fragmented in the cooling pool. A sluice gate separates the hopper from the grinder and flushing apparatus. This gate is opened to allow the flushing of the hopper contents. However, some clinkers get stuck before they reach the grinder and others are simply too large to be handled by the grinder. In addition, the strength properties of clinkers are anisotropic and unpredictable. The clearing of a particularly hard clinker that is sufficiently large requires a separate procedure. Currently the electric power industry uses a manual clinker clearing operation that is described below.
Once the flushing apparatus is obstructed, the hopper must be drained and the clinkers must be dislodged and broken into small enough pieces which the grinder can handle. This process usually requires the furnace to be shut down completely or to be operated at a reduced output. Once the hopper is drained, workers open an access port at the bottom of the hopper. The most common method of clearing the clinkers is to use a long heavy steel rod with a chisel on its end. A jackhammer is connected to the other end of the rod and the impacts are transmitted through the rod to a chisel at the end of the rod and onto the clinkers. This operation usually involves three to four workers. One worker stands at the top of a ladder or platform, which is at the level of the hopper access port, and positions the chisel. One or two workers support the rod, and another worker operates the jackhammer. The clinker clearing process is extremely dangerous since clinkers can drop from the hopper walls onto the pole as the workers are holding on to the pole. Pieces of molten clinkers can also come out of the open port. The danger is further intensified by the high temperature and poor visibility inside the hopper which is filled with a cloud of smoke and molten ash. The worker who is located on the top of the ladder is required to peer into the port to identify the current clinker location. The original clinker geometry is identified quite accurately by using a sonar mapping system when the hopper bottom is filled with water prior to flushing and opening of the port. The entire operation can take several hours to a day to complete which can spell large financial losses for the power plant
Since the environment inside the boiler is a hazardous one, the workers are required to wear cumbersome protective suits and head gear which can get very uncomfortable around the elevated temperatures inside the hopper. They also have to lift the heavy pole with the jackhammer while clearing the clinkers. Therefore, fine positioning of the device during the process is difficult to achieve. During the clearing process, sometimes portions of the hot water inside the furnace can spill out from the port and the workers in the protective suits have to continue the process while they are knee deep in water. Due to the danger involved in the clinker clearing operation and the long period of time required to complete it, a safer and more efficient method is highly desirable. Safety concerns and plant productivity have been the primary driving factors behind the efforts to automate the clinker clearing process. The automation of the clinker clearing process is characterized by the unique environment which exists inside an electric power plant furnace bottom hopper. This environment is hazardous, unpredictable, and cannot be modeled accurately due to poor visibility. These factors render the application of existing robotic devices unsuitable. Commercially available robots either do not possess sufficient force generation capability, or they are too large for the cluttered geometry of the region in and around the hopper. In addition, these devices require actuated joints with associated electronics to be operated inside the furnace. The highly corrosive molten ash of the coal boilers requires the use of minimal moving parts that are inexpensive and easily replaced inside the hopper.